Recovering gallium from metallic aluminum



Patented June 3, 1952 RECOVERING GALLIUM FROM METALLIC ALUMINUM Francis0. Frary, Oakmont, Pa., assignor to Aluminum Company of America,Pittsburgh, Pa.,

a corporation of Pennsylvania No Drawing. 7 Application January 23,1948,

' Serial No. 4,062

3 Claims.

This invention relates to the production of gallium, and relatesparticularly to recovering that metal from metallic aluminum.

Electrolytically reduced aluminum normally contains a trace of gallium,in addition to minor amounts of other impurities (chiefly iron andsilicon) and any alloying components which may have been added.Apparently the presence of gallium results from the fact that thealuminum is produced from alumina made from bauxite which containsgallium. The gallium is not entirely separated from the aluminum duringextraction of the aluminum oxide from the bauxite in the customary Bayerprocess, nor during subsequent electrolytic reduction of the alumina toaluminum. The metallic aluminum contains only traces of gallium, usuallyno more than 0.02 per cent. No commercially feasible process hasheretofore been available for recovering the gallium from the metallicaluminum. It is the principal object of this invention to provide apractical method of recovering gallium from aluminum.'

As is known in the art and disclosed in such U. S. patents as 673,364,1,534,317 to 1,534,320, inclusive, and 1,535,458, aluminum containingimpurities can be refined electrolytically (as distinguished from beingreduced electrolytically) by employing it in the molten metal anodelayer of an electrolytic cell in which the anode layer is separated froma cathode by a fused salt bath or electrolyte, and aluminum iselectrolytically dissolved from the molten anode .layer and deposited onthe cathode, leaving impurities behind in the anode layer. I have foundthat, in spite of its close chemical resemblance to aluminum, gallium isone of the impurities which remains in the anode layer, and that theamount of gallium in that layer gradually increases as more and morealuminum is refined in the cell.

Consequently, continued operation of the refining cell involvingreplenishment of the aluminum of the molten anode metal graduallyincreases the gallium' concentration of the anode metal. The gallium inthe anode metal can reach a concentration many times that of gallium inthe aluminum introduced into the cell before the other impuritiesdeposited in the anode layer increase to a point where the anode metalis no longer suitable for further use in the cell and must be replaced.In fact, to the best of my knowledge, it is the effect of concentrationof other impurities in the anode layer upon the quality of the refinedaluminum that is the practical limiting factor upon the concentration ofgallium in the anode layer. It will, of course, be understood thatreplenishment of the aluminum content of the anode layer may be eithercontinuous or intermittent, and for want of a better phrase, I shallrefer to such replenishment as successive additions of aluminum. Theimportant fact is that the larger the amount of galliumbearing aluminumrefined in the cell by passage of aluminum from the anode layer to thecathode, the greater will be the gallium concentration created in theanode layer. 1

The composition of the resulting anode alloy depends on such factors asthe original composition of the molten anode layer, the variousimpurities or alloying ingredients present in the metal which "has beenrefined in the cell, the amount of metal that has been refined, and theextent to which aluminum has been electrolyzed out of the alloy. As isbrought out in the patents mentioned above, during operation of the cellthe composition of the anode layer should be such that that layer willremain below the fused salt layer, and will adequately mobile under thetemperatureand current conditions maintained in the cell. The mostpractical anode layer is an aluminum-copper alloy such as is disclosedin Patent 1,534,317.

During operation of the cell, such impurities as iron and silicon, whichare normally present in small amounts in commercial aluminum, are knownto accumulate in the anode layer as successive additions of metal arerefined in the cell. Consequently, the anode alloy containing theaccumulated gallium contains aluminum, impurities separated from themetal refined, and ingredients such as copper which have been employedto provide the anode layer with characteristics desired in that layerduring the refining operation. The problem of economically recoveringgallium from such an alloy by dissolving it and thereafter extractinggallium from the solution is complicated by the necessity for using asolvent for that purpose which will attack the metal sufficientlyrapidly for practical operation and extract the gallium from it, and yetwill not at the same time dissolve appreciable amounts of other metalspresent which would interfere with'subsequent recovery of gallium fromthe solution.

In carrying out the present invention, gallium is dissolved from thesolidified anode alloy of the refining cell by means of an alkali metalhydroxide solution, particularly an aqueous sodium hydroxide orpotassium hydroxide 'solution, in spite of the fact that the anode alloycontains substantial'amounts of other metals, such as iron and silicon,which might be expected to form compounds with the gallium that wouldbestable' against alkali metal hydroxide solutions. Aluminum present inthe anode alloy also dissolvesrin the solution, forming alkali metalaluminate, but heavy metals such as copper in the anode alloy do notdissolve readily in the solution. A 5-40 per cent solution of thehydroxide is satisfactory for the purpose, and preterably the solutionis kept hot during treatment of the anode alloy. To facilitateextraction of the gallium it is desirable to comminute the solidifiedanode metal before immersing it in the alkali metal hydroxide solution.

Any convenient procedure for contacting the anode metal with thehydroxide solution can be used. For example, the metal may merely bekept immersed in the solution until the gallium in the alloy isdissolved.- However, as the attack of the solution on the metal slowsdown, it may be more eflicient to transfer the remaining undissolvedmetal to a fresh supply of solution; alternatively, the efficiency maybe increased by maintaining countercurren-t flow of the anode metal andthe alkali metal hydroxide solution.

After the gallium has been dissolved from; the anode alloy by means ofthe alkali metal hydroxide solution, metallic gallium canbe directlyprecipitated from the resultant solution by electrolysis. The fact thatthe gallium can be precipitated directly in metallic form from thesolution is important in reducing the cost of the metal. Prior to suchprecipitation of gallium,

undissolved materials present'may be removed from the solution, and if,desired, dissolved constituents which mightcontaminate the galliumproduced, or interfere with precipitation of the gallium (such as leadand zinc), may be precipitated from'thesolution. H

Ordinarily the anode alloy from the aluminumrefining ,cell contains avery much larger prop rtionof aluminum thanof galliu -m at the time ofthe alkali metal hydroxide solution treatment. Thealuminum alsodissolvesreadily in; the alkali metal hydroxide solution, forming alkali'metal aluminate and thus making a; large part of the alkali metalhydroxide unavailable for dissolving gallium, or for holdinggalliumin'solution during subsequent evaporation to increase theconcentration of gallium in the solution.- Consequently, prior toprecipitating gallium from the solution it may in some instances bedesirable to precipitate aluminum from the solution as' calciumaluminate by adding to the solution a calcium compound, such as hydratedlime or quick lime, prior to precipitating metallic gallium from thesolution. 'That procedure is described in detail in my U. S. patem pucaugn serial: No, 739,538, filed April 5, 1947, now" Patent No.2,582,376, issued January 15,; 1-952, and entitled Process'for ProducingMeta Little or no g'alliuinis precipitated from the solution along withthe calcium aluminate. Thereafter the concentration of gallium in thesolution can beincreased to a much greater extent than would otherwisebe the 7 case, either by using the solution to dissolvesolutio'n'thlls'fb'rmed. "I

As an example of the operation of the invention, metallic aluminumcontaining small amounts of iron and silicon and about 0.015 per cent ofgallium was refined by periodically introducing many successive batchesof such metal into the molten anode alloy in an electrolytic refiningcell of the general type described in U. S. Patent 1,534,320, in whichthe anode alloy employed initially contained almost 60 per cent ofaluminum, 40 per cent of copper, and a total of less than 0.5 per centof silicon, iron and other impurities. The fused salt bath layerfloating on theanode alloy of the cell was like that described andclaimed in U. S. Patent 1,534,317. Ultimately the anode alloy resultingfrom operation of the cell in the customary manner, in the refining ofsuccessive additions of aluminum, contained 40.44- per cent of aluminum,46.04 per cent of copper, 5.51 per cent of iron, 5.33 per cent ofsilicon, 0.685 per cent of gallium, and minute amounts of lead, zinc,manganese, titanium, and chromium. The anode alloy was then tapped fromthe cell and allowed to solidify.

A portion of it having the composition stated above, and ground to passa 60 mesh screen, was added to a 10 per cent aqueous solution ofcommercial flake sodium hydroxide containing sodium hydroxide equal inweight to the portion of alloy used. The mixture was" boiled for onehour, after which the solution was filtered to remove undissolved metalremaining. The resultant solution contained about 26 grams per liter ofdissolved aluminum, and 0.43 gram per liter of dissolved gallium, whichwas about 63 per cent of the gallium in the anode alloy used. Thesolutionwas then evaporated to about 30 per cent of its original volume;and placed in a glass containerprovided with two spaced nickelelectrodes extending 'into the solution. This electrolytic cell was thenoperated at a cathode current density of about 0.51 ampere per squareinch and 4 volts, while the electrolyte was main tained at about F.Substantially all of the gallium in the electrolyte was deposited on thecathode as metallic gallium in the course of 28 hours.

I claim: V v

1. The method or recovering gallium from metallic aluminum, comprisingadding aluminum containing gallium to a molten metal anode of anelectrolytic cell, electrodepositing' aluminum from the said anode on acathode of the said cell through a molten salt bath layer, wherebygallium in the aluminum added to the said anode accumulates in the anodemetal, subsequently dissolving gallium from the resultant anode metal bymeans of an alkali metal hydroxide; solution after'the gallium contentof the anodeme'tal has become higher in such operation of theelectrolytic cell than the gallium content of the said aluminum added tothe said anode, and thereafter pre- 'after the gallium content of theanode metal has become higher insuch operation ofthe elec- REFERENCESCITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,534,320 Hoopes Apr. 21, 19251,855,455 McCutcheon Apr. 26, 1932 OTHER REFERENCES A Treatise onChemistry, by H. E. Roscoe and Schorlemmer, vol. 2 (The Metals) 1913,pages The Journal of the American Chemical Society, vol. 40 (1918),July-December, page 1536,

Metals and Metallic Compounds, vol. IV, by U. R. Evans, 1923, pages 195,196, 206-208.

Information Circular No. 6401 of the U. S. Bureau of Mines, Dept. ofCommerce, Nov. 1930, page 4.

Handbook of Chemistry and Physics, 26th edition, pages 380, 381,Chemical Rubber Publish- 15 ing Co. 1942, 1943,

1. THE METHOD OF RECOVERING GALLIUM FROM METALLIC ALUMINUM, COMPRISINGADDING ALUMINUM CONTAINING GALLIUM TO A MOLTEN METAL ANODE OF ANELECTROLYTIC CELL, ELECTRODEPOSITING ALUMINUM FROM THE SAID ANODE ON ACATHODE OF THE SAID CELL THROUGH A MOLTEN SALT BATH LAYER, WHEREBYGALLIUM IN THE ALUMINUM ADDED TO THE SAID ANODE ACCUMULATES IN THE ANODEMETAL, SUBSEQUENTLY DISSOLVING GALLIUM FROM THE RESULTANT ANODE METAL BYMEANS OF AN ALKALI METAL HYDROXIDE SOLUTION AFTER THE GALLIUM CONTENT OFTHE ANODE METAL HAS BECOME HIGHER IN SUCH OPERATION OF THE ELECTROLYTICCELL THAN THE GALLIUM CONTENT OF THE SAID ALUMINUM ADDED TO THE SAIDANODE, AND THEREAFTER PRECIPITATING SUCH DISSOLVED GALLIUM FROM THERESULTANT SOLUTION.